You want the upper mount points of the harness to be parallel to the horizontal axis of the car. Whatever setup you fab, do not drop the upper straps too far below shoulder height as it will result in spine compression should an impact occur.

As for setups, it's not too difficult if you have the tools and material. A setup that I like a lot is what the guys at www.speedwaremotorsports.com have come up with. The bar mounts behind the seat to the seatbelt feeds on each side. It then has two braces coming from the bar that mount to the floor of the car. It's a pretty solid design and one you should aim at emulating if you go through with fabbing it yourself.

One thing to note though...harnesses are not exactly safe without a cage. It's one thing to use them as a method to stop you from sliding around in your seat but without a proper cage, don't think that you're suddenly safer. Using a harness instead of a normal seat belt, should a rollover occur, the harness will not allow your body to slide down off the seat thereby increasing the potential for head injury. Just something to think about.